3.建立数学模型： 求X1、X2、X3满足 X,+X2+X3 ≤95 6X1+5X2+2X3 ≤400 50X+20X2 ≤2000 12X,+10X2+16X3 ≤1200 X,20X220X320 使F=600X1+400X2+300X3 →Max